1. Field of the Invention
This invention relates to a device for magnetically detecting an angle of rotation of a rotary body. More particularly, the invention relates to a detection device well-suited for obtaining such basic information as, e.g. the ignition timing of an engine and is of the type which includes a plurality of magnets of different polarities arranged with one polarity alternately with the other polarity on the circumferential surface of a rotary body that rotates together with an engine crankshaft, and a magnetically sensitive element arranged to oppose these magnets, wherein the magnets on the rotary body are scanned by the magnetically sensitive element when the body is rotated, thereby an angle of rotation of the rotary body is detected.
2. Description of Prior Art
Electronic control has come to be widely employed in automotive vehicles in recent years. Since an item of basic information necessary for such electronic control is the rotation angle of the engine crankshaft, means are provided for detecting the rotation angle. One example of such a rotation angle detector includes magnets of different polarities arranged with one polarity alternately with the other polarity on the circumferential surface of a rotary body, and a rotation angle sensor spaced a predetermined distance from the circumferential surface of the rotary body. The magnets, which move together with the rotary body when the body is rotated, are scanned by the sensor, which produces an output signal indicative of the angle of rotation.
A specific example of such a rotation angle detector is disclosed in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 57-46117. One type of rotation angle detector 11 suggested by this reference has the structure shown in FIGS. 4 and 5.
The rotation angle detector 11 includes, for example, a cylindrical rotary body 13 mounted on and integrally with a rotary shaft 12 that rotates together with and engine crankshaft. Magnets r.sub.1, r.sub.3, r.sub.5, r.sub.7 . . . of N polarity and magnets r.sub.2, r.sub.4, r.sub.6, r.sub.8 . . . of S polarity are arranged on the circumferential surface of the rotary body 13 alternately of N and S polarities. The detector 11 further includes a rotation angle sensor 14, which comprises an element sensitive to the magnetic fields produced by the aforementioned magnets and which is arranged at a position spaced a predetermined distance from the circumferential surface of the rotary body 13.
The magnets r.sub.3, r.sub.4, r.sub.5, r.sub.6, of the magnets r.sub.1 -r.sub.8 on the portions of the circumferential surface of rotary body 13 are formed to be uniformly narrow in width (width being a length along the circumferential direction of the rotary body), while the remaining magnets r.sub.1, r.sub.2, r.sub.7, r.sub.8 are formed to be comparatively wide in width (width being as defined above). The narrow magnets r.sub.3 through r.sub.6 are used to produce a reference signal, whereas the wide magnets r.sub.1, r.sub.2, r.sub.7, r.sub.8 are employed to produce a rotation angle signal. For example, the width of each of the magnets r.sub.3 through r.sub.6 corresponds to an angle of .theta. with respect to the center O of rotation, and the width of each of the magnets r.sub.1, r.sub.2, r.sub.7, r.sub.8 corresponds to an angle of 2.theta..
With the rotation angle detector 11, the magnets r.sub.1 through r.sub.8 disposed on the circumferential surface of the rotary body 13 are scanned by the rotation angle sensor 14 as the rotary body 13 rotates, and the rotation angle sensor 14 produces electric signals corresponding to each width (.theta., 2.theta.) of the magnets r.sub.1 through r.sub.8. The output signal of the sensor 14 contains rotation angle signals corresponding to the wide magnets and reference signals corresponding to the narrow magnets, as shown in FIG. 6(c). As shown in FIG. 6(d), this output signal is shaped into a pulsed waveform by a waveform shaping circuit. The resulting pulses are used respectively as signals representing, for example, and angle of the crankshaft and as reference signals in controlling the ignition timing of an engine.
An advantage of the rotation angle detector 11 is that both the rotation angle signal and reference signal can be detected by the single rotation angle sensor 14. However, the inventors have discovered that the rotation angle detector of this type exhibits a magnetic imbalance at the boundaries between adjacent magnets of different widths arranged on the circumferential surface of the rotary body. The problem caused by this magnetic imbalance is that and angular error is produced in the shaped output waveform.
More specifically, if the magnets are scanned by using a Hall device as the rotation angle sensor 14, it is preferable that the ideal sensor output corresponding to the magnets r.sub.1 through r.sub.8 should appear as shown in FIG. 6(a). However, the actual sensor output takes the form shown in FIG. 6(c), in which errors are produced at points A and B where there is a change in magnet width. Whereas the ideal waveform following shaping should appear as depicted in FIG. 6(b), the actual shaped waveform contains angular errors, as illustrated in FIG. 6(d). It should be noted that the actual sensor output incurs a delay at the slice level at point A, where magnet width changes from wide to narrow, as a result of which the same delay appears in the actual shaped waveform. The phenomenon that occurs at point B, where magnet width changes from narrow to wide, is the reverse, thereby to raise the problem of failure to provide a precise engine control.
In view of the aforementioned problems in the prior art rotation angle detector, the present invention has been made to solve the problems effectively.